Titanium materials are light in weight, yet high in strength and excellent in corrosion resistance, so are being used even for the exhaust systems of automobiles. The combustion gas discharged from the engines of automobiles and motorcycles is collected at an exhaust manifold and discharged by an exhaust pipe from an exhaust outlet at the rear of a vehicle.
An exhaust pipe is formed split into several segments to enable insertion of a catalyst device which carries or is coated with a catalyst or of a muffler in the middle. In this Specification, the entire system from the exhaust manifold to the exhaust pipe and exhaust outlet will be called an “exhaust system”.
For the materials for such an exhaust system, in place of the conventional stainless steel material, from the viewpoint of reducing the weight of the vehicle, JIS class 2 commercially pure titanium material is being used—mostly for motorcycles. Furthermore, recently, in place of the JIS class 2 commercially pure titanium material, a titanium alloy with a higher heat resistance is used. Further, in recent years, to remove harmful ingredients from exhaust gas, mufflers which carry catalysts which are used at a high temperature are also being used. In addition, to obtain the most suitable structure when mounted in a vehicle, more severe bulging or drawing becomes necessary. Materials excellent in these become necessary.
The temperature of exhaust gas sometimes exceeds 700° C. and temporarily even reaches 800° C. For this reason, in materials which are used for exhaust systems, indicators such as the strength at a temperature around 800° C., oxidation resistance, creep speed at 600 to 700° C., and other aspects of high temperature heat resistance are stressed. Ti-3Al-2.5V alloy is excellent in the point of high temperature strength.
In PLT 1, a titanium alloy which is excellent in oxidation resistance and corrosion resistance is proposed.
In PLT 2, a heat resistant titanium alloy sheet which is excellent in cold workability and a method of production of the same are proposed.
In PLT 3, a titanium alloy with a surface covered by a protective film is proposed.
In PLT 4, a titanium alloy which is excellent in high temperature strength at 700° C. and oxidation resistance at 800° C. is proposed.